Levulinic acid, or 4-oxopentanoic acid, is an organic compound with the formula CH3C(O)CH2CH2CO2H. Levulinic acid is a precursor to pharmaceuticals, plasticizers, and various other additives. Also biofuels, such as methyltetrahydrofuran, gamma-valerolactone, and ethyl levulinate, can be prepared from levulinic acid.
Levulinic acid preferably is obtained as pure as possible, as this is preferred for the further use of levulinic acid for the production of pharmaceuticals, food additives and cosmetics. For these applications levulinic acid preferably has a purity of at least 95 wt. %, more preferably of at least 98 wt. %, based on the total weight of the levulinic acid. Also various chemical conversions of levulinic acid towards its derivatives suffer in selectivity and activity due to the presence of impurities in the levulinic acid.
Levulinic acid can be obtained by degradation of cellulose that is present in C6 carbohydrate-containing feedstocks, for example in agricultural waste products, waste from the paper industry or municipal waste.
In the prior art several processes for the production of levulinic acid are described.
The production of levulinic acid by acid catalyzed hydrolysis of C6 carbohydrate-containing feedstocks is described e.g. in WO2014/087016 A1 and US2010/312006. The described processes yield a biomass hydrolysate that contains next to the desired product, levulinic acid, several by-products that are derived during conversion of C6 carbohydrate-containing feedstocks by acid catalyzed hydrolysis. A process for the production of levulinic acid from C6 carbohydrate-containing feedstocks will thus generate a complex mixture wherefrom levulinic acid has to be recovered and purified.
The further purification of a biomass hydrolysate to a levulinic acid composition having a higher levulinic acid concentration is described in for example U.S. Pat. Nos. 5,608,105, 4,897,497, 6,054,611, WO2010/030617 and WO2014/087013.
WO2010/030617, for example, describes a purification via filtration, extraction and distillation to a higher concentrated levulinic acid composition. This levulinic acid composition is not of sufficient purity and thus not suitable for many applications mentioned above.
Purification of a biomass hydrolysate to a concentrated levulinic acid composition by the use of only distillation is, for example, mentioned in WO2014/087013. It is known to a person skilled in the art that distillation of levulinic acid always yields small amounts of angelical lactone which is known to cause coloration and low shelf time stability of levulinic acid. This is undesirable which is in particular addressed in U.S. Pat. No. 2,780,588.
Other methods for the purification of a concentrated levulinic acid composition are, for example, mentioned in U.S. Pat. Nos. 2,305,738, 2,029,412, 2,684,982, 2,349,514. These documents hint toward the possibility to purify a concentrated levulinic acid composition via crystallization. In the mentioned documents it is, however, not specified how crystallization should be performed or what type of crystallization can be used for the purification of levulinic acid.
The purification of a levulinic acid solution to a purity above 99 wt. % is mentioned in U.S. Pat. Nos. 8,507,718 and 9,073,841. These patent publications do not specify the method of crystallization either.
US2014/0316161 mentions crystallization from a dilute (10-50 wt %) levulinic acid composition in a non-specified solvent as a method to obtain pure levulinic acid. The use of diluted levulinic acid is undesirable due to the low temperatures necessary for crystallization of levulinic acid from a diluted levulinic acid. Further, the use of a solvent is undesirable as it brings additional complexity to the purification process.
Crystallization from solution and melt crystallization are, in general, two commonly used processes for the purification of chemical products.
Typical of crystallization from solution is that a solvent is the major compound in a solution that is fed to the crystallization unit. A disadvantage of crystallization from solution is that the impure levulinic acid product needs to be dissolved in solvent, crystallizes in this solvent and, thereafter, recovery of the solvent is required.
Melt crystallization is normally applied for products wherein the crystallizing component is present in a higher amount in the composition than all of the other components together.
Melt crystallization however is considered very unpredictable by the person skilled in the art. This is because the result of melt crystallization does not only depend on the freezing point of the product, the nature of impurities and whether the impurities form an eutectic mixture with the product, but also on the structure of the crystals formed and therefore on their tendency to occlude impurities. Moreover, the size and productivity of the melt crystallization depend on the rate of crystal formation without the occlusion of those impurities. In a review of Wynn (Separate organics by melt Crystallization, Chemical Engineering Progress, March 1992, 52-60) was stated “Unfortunately, in melt crystallization, the critical steps are rate dependent. They cannot be predicted accurately from theory. Laboratory or pilot plant data must be generated before even process feasibility can be established”.
The man skilled in the art is led away from the use of crystal seeds for the crystallization of levulinic acid, because in a review article of Jiang et al.; Research progress and model development of crystal layer growth and impurity distribution in layer melt crystallization, IE&C Research, August 2014; it is stated that “commonly industrial configurations of melt crystallization are towers with parallel tubes. Because of this, it is difficult to add and disperse crystal seeds. If seeds added are not distributed evenly along the tower unsatisfactory crystal layer growth may result in tube blocking. The most common method to activate the nucleus is implementing a high super cooling degree on the cold tube surface without seeds.”
In this article the wording ‘the activation of the nucleus’ describes the formation of the first crystals to start the melt crystallization process; this is also called the nucleation.